HRTEM observation of silicides and Laves phase precipitates in Nb-Ti-Si based alloys

The morphologies, crystallographic characteristics and formation processes of silicides and Laves phase precipitates in Nb-Ti-Si based alloys have been systematically investigated by high-resolution transmission electron microscopy (HRTEM) observation. Silicide precipitate of delta Nb11Si4 usually exhibits homogeneous distribution with acicular and tetrapod morphologies, possessing uniform orientation relationships (ORs) with Nbss. The occurrence of these morphologies might be caused by the symmetry decrease during the phase transformation of Nbss -> delta Nb11Si4 + Nbss(1). In some cases, the delta Nb11Si4 precipitates also adopt heterogeneous distribution with plate morphology and identical ORs as tetrapod precipitates with Nbss. The Laves phase precipitate, Cr2Nb, possesses C15 structure and contains high densities of stacking faults (SFs). It always forms along defects or Nbss/gamma Nb5Si3 interfaces. The coupling precipitation behaviors and precipitation reactions of delta Nb11Si4, Cr2Nb and gamma Nb5Si3 in the Nb-Ti-Si based alloys have been discussed.